Friday, February 20, 2009

Cardiac Muscle

Cardiac muscle is a type of striated muscle found in the walls of the heart, specifically the myocardium. Cardiac muscle cells are known as cardiac myocytes. The cells that comprise cardiac muscle are sometimes seen as intermediate between skeletal and smooth muscle in terms of appearance, structure, metabolism, excitation-coupling and mechanism of contraction. It is under control of the autonomic nervous system. The central nervous system does not directly create the impulses to contract the heart, but only sends signals to speed up or slow down the heart rate through the autonomic nervous system using two opposing kinds of modulation: 1) sympathetic nervous system; 2) parasympathetic nervous system.

Cardiac muscle shares similarities with skeletal muscle with regard to its striated appearance and contraction, with both differing significantly from smooth muscle cells. Coordinated contraction of cardiac muscle cells in the heart propel blood from the atria and ventricles to the blood vessels of the circulatory system. Cardiac muscle cells, like all tissues in the body, rely on an ample blood supply to deliver oxygen and nutrients and to remove waste products such as carbon dioxide. The coronary arteries fulfill this function.

Cardiac muscle is adapted to be highly resistant to fatigue: it has a large number of mitochondria, enabling continuous aerobic respiration via oxidative phosphorylation, numerous myoglobins and a good blood supply, which provides nutrients and oxygen. In contrast to skeletal muscle, cardiac muscle requires both extracellular calcium and sodium ions for contraction to occur. Like skeletal muscle, the initiation and upshoot of the action potential in cardiac muscle cells is derived from the entry of sodium ions across the sarcolemma in a positive feedback loop.